Back contact electrodes for cadmium telluride photovoltaic cells

ABSTRACT

A method for forming a back contact for a photovoltaic cell that includes at least one semiconductor layer is provided. The method includes applying a continuous film of a chemically active material on a surface of the semiconductor layer and activating the chemically active material such that the activated material etches the surface of the semiconductor layer. The method further includes removing the continuous film of the activated material from the photovoltaic cell and depositing a metal contact layer on the etched surface of the semiconductor layer.

PRIORITY INFORMATION

The present application claims priority to and is a divisionalapplication of U.S. patent application Ser. No. 12/765,225 titled“Methods for Forming Back Contact Electrodes for Cadmium TelluridePhotovoltaic Cells” of Korevaar, et al. filed on Apr. 22, 2010(published as US 2011/0259423), which is incorporated by referenceherein.

BACKGROUND

The invention relates generally to photovoltaic cells and, moreparticularly, to methods for forming back contact electrodes forphotovoltaic cells.

Solar energy is abundant in many parts of the world year round. Thus,photovoltaic (PV) devices, which convert solar energy into electricalenergy, have the potential to provide a reliable form of clean,renewable energy in many parts of the world. Typically, in its basicform, a PV (or solar) cell includes a semiconductor junction made of twoor three layers that are disposed on a substrate layer, and two contacts(electrically conductive layers) for passing electrical energy in theform of electrical current to an external circuit. Moreover, additionallayers are often employed to enhance the conversion efficiency of the PVdevice.

There are a variety of candidate material systems for PV cells, each ofwhich has certain advantages and disadvantages. Cadmium telluride (CdTe)is a prominent polycrystalline thin-film material, with a nearly idealbandgap of about 1.45-1.5 electron volts. CdTe also has a very highabsorptivity, and films of CdTe can be manufactured using low-costtechniques. In theory, solar cell efficiencies in excess of twentypercent (20%) could be achieved for cadmium sulfide (CdS)/CdTe devices,provided various issues with the quality of the individual semiconductorlayers and with the back contact electrode can be overcome.

Because of the high work function of CdTe, conventional metal backcontacts are not generally viewed as being suitable. Instead, graphitepastes (either undoped or doped, for example with copper or mercury) arewidely used as a back contact for CdTe PV cells. However, thesegraphite-paste back contacts tend to degrade significantly over time, ascan be shown via accelerated lifetime testing. This degradationtypically manifests itself as a decrease in fill factor (FF) and/or opencircuit voltage V_(OC) over time. The fill factor degradation istypically driven by a decrease in shunt resistance (R_(sh)) and anincrease in the series resistance (R_(OC)) over time. The degradation ofthe back contact electrodes undesirably leads to degradation of thesolar cell efficiency, on a long-term basis.

To date, the failure to develop low-resistance contacts has hindered thecommercialization of CdTe solar cells. A cost-effective solution to thisproblem would remove one of the remaining hurdles for commercializingCdTe photovoltaic modules.

It would therefore be desirable to provide a back contact electrode fora CdTe PV cell, which exhibits less degradation over the lifetime of thePV cell. It would further be desirable to provide an economical methodfor forming the improved back contact electrode, in order to facilitatecommercialization of CdTe PV cells.

BRIEF DESCRIPTION

One aspect of the present invention resides in a method for forming aback contact for a photovoltaic cell that includes at least onesemiconductor layer. The method includes applying a continuous film of achemically active material on a surface of the semiconductor layer andactivating the chemically active material, such that the activatedmaterial etches the surface of the semiconductor layer. The methodfurther includes removing the continuous film of the activated materialfrom the photovoltaic cell and depositing a metal contact layer on theetched surface of the semiconductor layer.

Another aspect of the present invention resides in a method for forminga back contact for a photovoltaic cell that includes at least onecadmium telluride (CdTe) layer. The method includes applying acontinuous film of a chemically active material on a surface of the CdTelayer and activating the chemically active material such that thesurface of the CdTe layer is enriched with tellurium (Te). The methodfurther includes removing the continuous film of the activated materialfrom the photovoltaic cell and depositing a metal contact layer on theTe enriched surface of the CdTe layer.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a first processing step for forming a back contactelectrode;

FIG. 2 illustrates a subsequent processing step for forming a backcontact electrode;

FIG. 3 illustrates an example back contact electrode structure;

FIG. 4 illustrates a metal treatment step, where the metal may bedeposited as a metal or as a metal-salt;

FIG. 5 illustrates an example method for forming a back contact for aphotovoltaic cell;

FIG. 6 illustrates an additional optional processing step for the methodshown in FIG. 5;

FIG. 7 illustrates an additional optional processing step for the methodshown in FIG. 5;

FIG. 8 illustrates another example back contact electrode structure; and

FIG. 9 is a graph showing the results of temperature acceleratedlifetime measurements for different types of back contact electrodes.

DETAILED DESCRIPTION

As noted above, the high work function of CdTe leaves a relatively smallset of metals that can be employed to form an ohmic contact with theCdTe layer. Suitable metals include platinum and gold, which are notcommercially viable candidates for low-cost CdTe PV cells. However,lower cost metals, such as molybdenum, nickel, and chromium, typicallyform a tunneling barrier at the interface between the back contact andthe CdTe layer. As CdTe typically has carrier densities between about1×10¹⁴ to about 1×10¹⁵ per cubic centimeter, this tunneling barrier maybe relatively large. Thus, absent proper treatment of the backside ofthe CdTe layer, the resistance with the back contact layer can besignificant, thereby reducing the fill factor (and hence the efficiency)of the PV cell.

The present invention addresses these issues, and a method for formingan improved back contact 12 for a photovoltaic cell 10 is provided. Themethod is described with reference to FIGS. 1-8. As indicated, forexample, in FIG. 3, the photovoltaic cell 10 includes at least onesemiconductor layer 14. For the illustrated arrangements, thesemiconductor layer 14 comprises cadmium telluride (CdTe). Although theexamples provided are directed to CdTe, the invention can be used toform improved back contact electrodes for other semiconductors as well.Other example materials for semiconductor layer 14 include, withoutlimitation, CdZnTe and ZnTe.

As indicated, for example, in FIGS. 1 and 5, the method for forming aback contact for a photovoltaic cell includes, at step 30, applying acontinuous film 16 of a chemically active material on a surface 15 ofthe semiconductor layer 14. For certain example arrangements, thecontinuous film 16 has a thickness, which is greater than or equal to0.5 μm. For more particular examples, the thickness of the continuousfilm is in a range of about 1-10 μm, and more particularly of about 2-5μm. The continuous film 16 may be deposited using a variety oftechniques, non-limiting examples of which include printing,screen-printing, dipping and spray coating.

As indicated in FIG. 5, the method further includes, at step 32,activating the chemically active material, such that the activatedmaterial etches the surface 15 of the semiconductor layer 14. Forparticular examples, the chemically active material comprises achemically active paste. As used here, the term “paste” should beunderstood to mean a plurality of particles suspended in a binder.Non-limiting examples of the chemically active paste include pastescontaining high surface area solid particles, such as carbon, silica oralumina, dispersed therein and also optionally containing halogen atoms,such as in the form of hydrochloric acid, chloride salt or iodide saltor organic compounds typically used for ion exchange resins. Althoughchlorine and iodine are listed as non-limiting examples, the chemicallyactive paste may contain other halogens.

For more particular examples, the chemically active paste comprises aplurality of particles selected from the group consisting of alumina,carbon, boron nitride, silica and combinations thereof and a binder,where the particles are suspended in the binder. Non-limiting examplesof the binder include water and organic solvents, such as dimethylesters of dicarboxylic acids. It will be recognized that a variety ofbinders may be used, and the invention is not limited to a specificbinder.

For certain examples the chemically active material comprises a graphitepaste. The graphite paste may be undoped or may be doped with elementsfrom groups IB or IIB of the periodic table of elements and/or elementsfrom group VIA, such as, but not limited to copper (Cu), mercury (Hg),gold (Au), silver (Ag), zinc (Zn), tellurium (Te), selenium (Se), andcompounds and combinations thereof, such as Cu/Au— or Cu/HgTe, Ag₂Te,Ag₃PO₄, and Ag₂MoO₄, as well as antimony—(Sb), Nickel—(Ni),bismuth—(Bi), and lead—(Pb) based compounds. For certain examples,copper may be introduced into the graphite paste in the form of a classIB-VIA or IIB-VIA or IB-IIB-VIA semiconductor compound, such as coppertelluride, mercury telluride, or as a copper dopant in mercurytelluride. Beneficially, the introduction of copper in the form of aclass IB-VIA or IB-IIB-VIA semiconductor compound, instead of usingelemental or free copper, may help to prevent the diffusion of copperinto the semiconductor layer 14.

For particular embodiments, the activation step 32 for the continuousfilm 16 comprises annealing the photovoltaic cell. In one non-limitingexample, the photovoltaic cell 10 may be annealed at a temperature in arange of about 100-300 degrees Celsius, for a period in a range of about5-30 minutes. However, it will be recognized that the specific annealtemperatures and times will vary based on the thickness of the film 16and on the composition of the chemically activated material. Inaddition, although thermal activation is the example means foractivating a graphite paste, other materials may be activated by othermeans, such as light, and the present invention is not restricted to aspecific activation means.

According to more particular embodiments, performing the step 32 ofactivating the chemically active material makes the surface 15 of theCdTe layer 14 tellurium (Te)-rich. For particular examples, the surface15 of the CdTe layer 14 has a Te/Cd ratio greater than about two (2).According to a more particular embodiment, the ratio Te/Cd>3 at thesurface (15) of the CdTe layer 14, and still more particularly, Te/Cd>5at the surface 15 of the CdTe layer 14. The Te enrichment is the highestat the surface 15 and tapers off as one moves deeper inside the CdTelayer 14, such that Te/Cd=1 at a certain depth within the CdTe layer 14.The relative concentration of Te to Cd at the surface 15 of the CdTelayer 14 can be measured, for example using X-ray PhotoelectronSpectroscopy (XPS) and to some extent using Time-Of-Flight Secondary IonMass Spectroscopy (TOF-SIMS).

Returning to FIG. 5, the method further includes, at step 34, removingthe continuous film 16 of the activated material from the photovoltaiccell 10. For particular examples, performing the step 34 of removing thecontinuous film 16 of the activated material from the photovoltaic cell10 comprises applying a solvent to the activated material and removingthe activated material and the solvent from the photovoltaic cell 10. Inone non-limiting example, the continuous film 16 comprises a graphitepaste, and the solvent comprises dimethylformamide (DMF). Other examplesolvents include water, tetrahydrofuran (THF), and methylethylketone(MEK). However, the invention is not limited to the use of specificsolvents, but rather the specific solvent selected will vary based uponthe specific chemically active material being removed and based uponother requirements, such as environmental and cost considerations. Forcertain implementations, the activated material and solvent are removedusing non-contact means, such as an air or liquid jet. However, in otherimplementations, contact means could also be used, for example thepartially dissolved material could be scraped or wiped off. For otherexample configurations, the step of applying a solution is omitted, andthe continuous film 16 is removed by physical means, for example using alaser. It should be recognized that these examples are merelyillustrative, and the invention is not limited to a specific techniquefor removing the continuous film.

As indicated, for example, in FIGS. 2 and 5, the method for forming aback contact for a photovoltaic cell further includes, at step 36,depositing a metal contact layer 18 on the etched surface 15 of thesemiconductor layer. For particular configurations, the metal contactlayer 18 comprises a metal selected from the group consisting ofmolybdenum, tantalum, tungsten, alloys of molybdenum, tantalum, titaniumor tungsten, compounds comprising molybdenum or tungsten (e.g.molybdenumnitride), and combinations thereof. In one non-limitingexample, the metal contact layer has a thickness of less than or equalto 100 nm. The metal contact layer 18 may be deposited using a varietyof techniques, non-limiting examples of which include evaporation andsputtering. Moreover, one or more additional metal layers may bedisposed on the metal contact layer 18 to form the back contactelectrode 12. For example and as indicated in FIG. 3, an aluminum layer19 with a thickness in a range of 50-1000 nm may be deposited on amolybdenum contact layer 18. For specific arrangements and as indicatedin FIG. 3, a nickel layer 21 with a thickness in a range of 20-200 nmmay be disposed on the aluminum layer 19. For this arrangement, the backcontact electrode 12 comprises the molybdenum 18/aluminum 19/nickel 21stack. In another example, the nickel—is replaced with chromium 21 withan example thickness range of 20-100 nm, such that the back contactelectrode 12 comprises the molybdenum 18/aluminum 19/chromium stack 21.

In addition and as indicated in FIGS. 4 and 6, for example, the methodof forming a back contact may optionally include, at step 38, depositinga metal layer 22 on the surface 15 of the semiconductor layer 14 priorto applying the continuous film 16. For this embodiment, at step 30 acontinuous film of a chemically active material is deposited on themetal layer 22. Non-limiting examples of suitable materials for themetal layer 22 include copper, gold, silver, zinc and mercury. Examplethickness for the metal layer 22 is in a range of about 0.1-10 nm. Themetal layer 22 may be applied using a variety of techniques,non-limiting examples of which include evaporation and sputtering. Inone non-limiting example, a very thin Cu-layer 22 is deposited at step38 on the surface 15 by evaporation, and this process is typicallyreferred to as a Cu-treatment step. Beneficially, ‘intermixing’ willthen occur during the Te-enrichment step. Process steps 32, 34 and 36are then performed, as indicated in FIG. 6. Although not shown in theillustrations, under certain processing conditions, some portion of themetal layer 22 may remain at the surface 15 of the semiconductor layer14. However, the process parameters may be selected such that all of themetal reacts with the CdTe layer 14, such that none of the metal layer22 remains at this interface.

FIGS. 4 and 6 also illustrate an example method for forming a backcontact that optionally includes, at step 38, depositing a metal saltsolution (also indicated by reference numeral 22 in FIG. 4) on thesurface 15 of the semiconductor layer 14 prior to applying thecontinuous film 16. In one non-limiting example, a Cu-salt solution 22is sprayed on the surface 15 of the semiconductor layer (14). Although22 is depicted as a layer in FIG. 4, one skilled in the art willrecognize that the metal-salt spray technique will not result in acontinuous metal film. For this process, at step 30, a continuous filmof a chemically active material is deposited after the Cu-salt solution22 has been sprayed on the surface 15 of the semiconductor layer.Process steps 32, 34 and 36 are then performed, as indicated in FIG. 6.

FIGS. 4, 7 and 8 illustrate an example method for forming a back contactthat optionally includes, at step 40, depositing a metal layer 22 on thesurface 15 of the semiconductor layer 14 after the continuous film 16has been removed (at step 34) and prior to applying the metal contactlayer 18 (at step 42). For the resulting arrangement shown in FIG. 8,the back contact electrode 12 also includes optional metal layer 22. Forparticular configurations, the metal in layer 22 is selected from IB orIIB groups, such as Cu, Au, Ag, Hg, Zn or a combination thereof.

In addition to the optional process steps discussed above with referenceto FIGS. 6 and 7, metal, such as copper, gold, silver, zinc and mercury,may be introduced by adding the respective metal particles, in eitherelemental or compound form, to the chemically active material prior toits application to the surface 15 of the semiconductor layer 14.

Beneficially, the above-described method provides back contactelectrodes for CdTe PV cells, which exhibit less degradation over thelifetime of the PV cell, as demonstrated by the following example.

EXAMPLE

FIG. 9 illustrates the performance of three types of back contactelectrodes for CdTe PV cells that were subjected to temperatureaccelerated lifetime measurements. In particular, FIG. 9 showstemperature accelerated open circuit voltage (V_(oc)) measurementsobtained for CdTe cells with three different types of back contactelectrodes. After fabrication, the CdTe PV cells were placed in anaccelerated lifetime (“ALT”) tester, which exposed the devices to acontinuous illumination level of 0.7 sun, at a temperature of 65 C,under open circuit conditions. Periodically, the devices were taken outof the ALT-tester, and their performance was measured under standardAM-1.5 conditions. FIG. 9 shows the resulting data, plotted as afunction of the time that the devices were exposed in the ALT-tester.

Three processes were employed to fabricate back contact electrodes forCdTe PV cells. For all three processes, the molybdenum/aluminum backcontact electrodes had a 100 nm thick Mo layer (layer 18), followed by100-1000 nm of Al (layer 19).

In the first process, the back-contact of the CdTe was treated withCdCl₂ and then Te-enriched by exposure to a nitric-phosphoric (NP) etchfor two minutes. Next, a copper-salt was sprayed onto the etchedsurface, and the resulting structure was baked. Next, Mo/Al back-contactmetal layers were deposited.

In the second process, the CdTe was sprayed and baked with a Cu-salt andthen covered with a graphite paste. A heat treatment was performedbefore the graphite was covered with the Mo/Al back-contact layers 18,19.

A third set of molybdenum/aluminum back contacts were formed using themethod of the present invention. First, a Cu-salt was sprayed onto asurface 15 of the CdTe layer 14, which was then baked. Next, acontinuous film 16 of a graphite paste was formed on the surface 15 ofthe CdTe layer 14. The graphite paste was then activated and removedfrom the photovoltaic cell 10 using DMF. The Mo/Al contact layer 18, 19was then deposited on the surface 15 of the semiconductor layer exposedby the graphite removal process. The molybdenum contacts were 100 nmthick followed by 100 nm of Al.

The three sets of PV cells were subjected to a continuous illuminationlevel of 0.7 sun, at a temperature of 65 C, under open circuitconditions for a period of 9 weeks, and open circuit voltagemeasurements were performed weekly. As shown in FIG. 9, the Mo/Al backcontacts formed using the method of the present invention exhibitedsignificantly lower degradation during the accelerated lifetime testingthan either the graphite contact electrodes or the Molybdenum backcontact electrodes formed after the NP-type etch (“NP” in FIG. 9).

In addition, there was a wide range of performance for the NPelectrodes, with the best NP back contact electrodes exhibitingperformance superior to that of the graphite contact electrodes, andwith the worst NP back contact electrodes degrading more rapidly thanthe graphite back contact electrodes, as shown in FIG. 9.

Although the cause of V_(oc), degradation of CdTe solar cells iscomplex, one possible theory is that V_(oc), degradation appears to be afunction of Te-richness and Cu-content. For the ‘loaded’ graphitecontacts, the significant degradation of V_(oc), shown in FIG. 9 may bedue to continuous interaction between etchants/impurities in thegraphite and the rest of the solar cell (primarily interaction betweenthe etchants/impurities in the graphite with the CdTe). By removing thechemically active film 16 prior to depositing the metal back contact,the present invention significantly reduces these interactions, therebyslowing the V_(oc), degradation of the resulting CdTe cells. For the“NP” contacts, grain-boundary attack of the NP etchant appears to causeshunting paths in the device, negatively impacting the V_(oc). Thepresent invention avoids this grain-boundary etching, because the pastedoes not penetrate into the grain-boundaries, as solutions, like NP, do.In addition, continuous exposure to the paste (as occurs for thegraphite contacts) will eventually start etching the grain-boundaries,likely due to vapors forming in the spaces between the paste and thegrain-boundaries.

A particular method for forming an improved back contact 12 for aphotovoltaic cell 10 that includes at least one Cadmium Telluride (CdTe)layer 14 is described with reference to FIGS. 1-8. As shown for examplein FIGS. 1 and 5, the method includes, at step 30, applying a continuousfilm 16 of a chemically active material on a surface 15 of the CdTelayer 14. The method further includes, at step 32, activating thechemically active material, such that the Tellurium (Te) is enriched onthe surface 15 of the CdTe layer 14. The method further includes, atstep 34, removing the continuous film 16 of the activated material fromthe photovoltaic cell 10 and, at step 36, depositing a metal contactlayer 18 on the Te enriched surface 15 of the CdTe layer 14.

According to a particular embodiment, the surface of the CdTe layer 14has a Te/Cadmium (Cd) ratio greater than about two (2). According to amore particular embodiment, Te/Cd>3 at the surface 15 of the CdTe layer14, and still more particularly, Te/Cd>5 at the surface 15 of the CdTelayer 14.

According to a particular embodiment, the chemically active materialcomprises a plurality of particles selected from the group consisting ofalumina, carbon, boron nitride, silica and combinations thereof and abinder, where the particles are suspended in the binder to form achemically active paste.

According to a particular embodiment, the step 32 of activating thecontinuous film 16 comprises annealing the photovoltaic cell, and themetal contact layer 18 comprises a metal selected from the groupconsisting of molybdenum, tantalum, tungsten, alloys of molybdenum,tantalum and tungsten, compounds comprising molybdenum or tungsten (e.g.molybdenum nitride), and combinations thereof. For this particularembodiment, the step 34 of removing the continuous film 16 of theactivated material from the photovoltaic cell 10 comprises applying asolvent to the activated material and removing the activated materialand the solvent from the photovoltaic cell 10.

As shown for example in FIGS. 4 and 6, the method optionally includes,at step 38, depositing a metal layer 22 on the surface 15 of the CdTelayer 14 prior to applying the continuous film 16.

As shown for example in FIG. 7, the method optionally includes, at step40, depositing a metal layer (or a metal salt solution) 22 on thesurface 15 of the CdTe layer 14 after the continuous film 16 has beenremoved and prior to applying the metal contact layer 18. An exampleresulting back contact is shown in FIG. 8.

For particular embodiments, the photovoltaic cell 10 with a back contact12 made using the above-described methods may have the followingdesirable properties. For certain configurations, a surface 15 of theCdTe layer 14 has a tellurium (Te)/cadmium (Cd) ratio greater than abouttwo (2). According to a more particular embodiment, Te/Cd>3 at thesurface 15 of the CdTe layer 14, and still more particularly, Te/Cd>5 atthe surface 15 of the CdTe layer 14. For particular embodiments, theCdTe layer 14 is Te enriched only at the surface 15 and not along thegrain-boundaries

Beneficially, the resulting PV cells have back contact electrodes, whichexhibit less degradation over the lifetime of the PV cell. By improvingthe stability of the back contact for the PV cells, higher outputs areachieved over the lifetime for the PV cells, as well as higher end oflife-efficiency for the PV devices.

Although only certain features of the invention have been illustratedand described herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

The invention claimed is:
 1. A photovoltaic cell, comprising: asemiconductor layer comprising cadmium telluride and defining an etchedsurface, wherein the etched surface of the semiconductor layer has atellurium to cadmium ratio that is greater than about 2; and a metalcontact layer deposited directly onto the etched surface; wherein thesemiconductor layer has a tellurium to cadmium ratio that is highest atthe surface and tapers off while moving deeper inside the semiconductorlayer such that the tellurium to cadmium ratio is 1 at a certain depthwithin the semiconductor layer.
 2. The photovoltaic cell as in claim 1,wherein the etched surface of the semiconductor layer has a tellurium tocadmium ratio that is greater than
 3. 3. The photovoltaic cell as inclaim 1, wherein the etched surface of the semiconductor layer has atellurium to cadmium ratio that is greater than
 5. 4. The photovoltaiccell as in claim 1, wherein the metal contact layer comprises a metalselected from the group consisting of molybdenum, tantalum, tungsten,alloys of molybdenum, tantalum, titanium or tungsten, compoundscomprising molybdenum or tungsten, and combinations thereof.
 5. Thephotovoltaic cell as in claim 1, wherein the metal contact layer has athickness of less than or equal to 100 nm.
 6. The photovoltaic cell asin claim 1, further comprising: a second metal contact layer depositedonto the metal contact layer.
 7. The photovoltaic cell as in claim 6,wherein the metal layer comprises molybdenum, and the second metal layercomprises aluminum.
 8. The photovoltaic cell as in claim 7, wherein thesecond metal layer has a thickness of 50-1000 nm.
 9. The photovoltaiccell as in claim 7, further comprising: a third metal layer deposited onthe second metal contact layer.
 10. The photovoltaic cell as in claim 9,wherein the third metal layer comprises nickel.
 11. The photovoltaiccell as in claim 9, wherein the third metal layer comprises chromium.12. The photovoltaic cell as in claim 9, wherein the third metal layerhas a thickness of 20-200 nm.
 13. A photovoltaic cell with a backcontact made using a method for forming a back contact for aphotovoltaic cell that includes at least one cadmium telluride (CdTe)layer, the method comprising: applying a continuous film of a chemicallyactive material on a surface of the CdTe layer; activating thechemically active material such that the surface of the CdTe layer isenriched with tellurium (Te); removing the continuous film of theactivated material from the photovoltaic cell; and depositing a metalcontact layer on the Te enriched surface of the CdTe layer; wherein theCdTe layer has a tellurium to cadmium ratio that is highest at thesurface and tapers off while moving deeper inside the semiconductorlayer such that the tellurium to cadmium ratio is 1 at a certain depthwithin the semiconductor layer.
 14. The photovoltaic cell of claim 13,wherein a surface of the CdTe layer has a tellurium (Te)/cadmium (Cd)ratio greater than about two.
 15. The photovoltaic cell of claim 13,wherein said CdTe layer further comprises a plurality of grainboundaries, wherein the CdTe layer is not Te enriched along thegrain-boundaries.
 16. The photovoltaic cell as in claim 1, wherein saidsemiconductor layer further comprises a plurality of grain boundaries,and the semiconductor layer is not Te enriched along thegrain-boundaries.